Polymer dielectrics have a great deal of potential in the electronics industry. They are generally cheaper and easier to work with than the corresponding inorganic materials. In addition, newer ultra-high density, high-speed circuity such as multichip modules (MCM) and chip-on-board (COB) printed circuit boards and new uses in liquid crystal displays (LCD's) require higher levels of performance than existing inorganic dielectrics can provide. There is also a desire to replace inorganic dielectrics with organic materials in integrated circuit (IC) applications because of the performance advantages that organic materials offer. While polymers have had little success in displacing established inorganics such as SiO.sub.2 in the semiconductor industry, the potential performance and cost advantages that organic systems offer are driving IC manufacturers to consider them for future generations of semiconductor products. However, despite the potential that organic materials offer, their widespread usefulness has been hindered by the lack of appropriate organic materials with the right combination of properties for these applications.
Polymers are needed with thermooxidative and thermomechanical stability sufficient to withstand 300.degree.-350.degree. C. processing steps, moisture absorptions in the 0.1-1% regime, thermal expansion characteristics which match inorganic substrates (and therefore exhibit low residual stress), and dielectric constants below 3.0. Low dielectric constant materials are especially important for the construction of future signal processing devices which are projected to have clock speeds operating at frequencies within the GHz-range. The new dielectrics must form uniform, high-quality coatings, exhibit excellent resistance to dissolution and crazing by common processing solvents, and show good adhesion to inorganic and metallic substrates. An ideal material would also be easy to process (e.g., spin coat reproducibly, with a minimum of effort) and exhibit excellent long-term stability in solution. This highly demanding combination of properties is not currently available in any commercial product.
Polyimides are the current "state-of-the-art" organic dielectric coatings for microelectronic packaging. Although polyimides exhibit some of the aforementioned attributes, they are not an ideal class of polymers for electronic applications. Some of the drawbacks inherent in polyimides may be attributed to the highly polar carbonyl groups, four of which are present in the repeat unit of typical polyimides. These carbonyls are believed to be responsible for the tendency of polyimides to absorb water, generally at values between 1% and 2% of their weight. Water absorption has a significant deleterious effect on the electrical properties of polyimides, e.g., increased dielectric constant and dielectric loss. In addition, the polarizable nature of the carbonyls is responsible for the relatively high dielectric constants exhibits by polyimides, when compared to less polar polymers.
Another drawback to polyimides for electronic coating applications is that they are usually marketed in a prepolymer form, typically as a polyamic acid solution. These lacquers are applied by spin coating followed by thermal imidization. Polyamic acid solutions are inherently unstable and the viscosities of these solutions can change unless great care is taken during their storage and transport, making the goal of obtaining reproducible spun-on polyimide coatings more difficult. In addition, the imidization process involves the evolution of small molecules (usually water) and significant changes in the chemical structure of the polymers occur. Thus, the conversion of the prepolymer to the final polymer undoubtedly contributes to the high degrees of residual stress often observed at the interface between silicon substrates and polyimide coatings.
The achievement of very low dielectric constants coupled with low moisture uptake in thermally stable polymers suitable for use as organic dielectrics requires polymers that are less polar than polyimides. A particularly well-suited class of polymers for this purpose are polyquinolines. Polyquinolines were developed by John K. Stille, whose research group demonstrated the synthesis of dozens of polyquinoline derivatives.
The quinoline group itself has very high thermal stability. When combined with other thermally stable groups, examples of which include phenyl, phenylene, phenoxy, oxy, diphenylmethylene, hexafluoroisopropylidene, 9,9'-fluorenylidene, dimethylsiloxy, diphenylsiloxy, methylphenylsiloxy, or thio, highly thermally stable polymers result. High thermal stability is useful for producing parts, films, fibers, and other objects which must withstand hot environments, including engine components, supersonic aircraft structures, and electronic components subjected to soldering temperatures or high temperature processing such as in integrated circuit manufacture.
Polyquinolines typically have excellent electrical properties, including low dielectric constants, and low moisture absorption. Polyquinolines are therefore useful as electrical insulators, or dielectrics as for example in printed wiring boards, multichip modules, integrated circuits, electrical connectors, capacitors, wire coating and the like. Polyquinolines having good thermal stability and excellent electrical and mechanical properties are disclosed in U.S. Pat. Nos. 4,000,187 and 5,017,677 to Stille and 5,247,050 to Hendricks and in J. K. Stille, Macromolecules, 1981, 14, 870, all of which are incorporated herein by reference.
For many electronics applications it is necessary to apply a layer of dielectric material in a specific pattern. Polyquinolines may be patterned by photolithography using techniques known for patterning other polymers, such as polyimides. The photolithographic process is capable of yielding very fine features, however, it is a complex process, and therefore costly. The total yield is the product of the yield of each individual step, so that fewer steps are always desired. Many steps in the photolithographic process can be eliminated if the material to be patterned is itself photosensitive, and a separate photoresist is not necessary. FIGS. 1 and 2 contrast the processing steps that are needed to wet etch non-photoimageable dielectrics with those needed to wet etch photoimageable dielectrics.
In FIG. 1 a substrate 1 is coated with a dielectric 3 in process 12. A photoresist 5 is coated on top of the dielectric 3 in process 14. The photoresist is exposed in process 18 by irradiating with light of a suitable frequency 11 through a mask 13. The resist is developed in process 20. The underlying dielectric 3 is etched in process 22. Finally the resist is stripped in process 24 to give patterned dielectric 9. In many cases additional steps are needed. A barrier layer may be needed between the resist and dielectric layer adding a step to process 14. This barrier layer must be etched and stripped adding one step each to processes 20 and 24.
FIG. 2 shows the corresponding steps for a photoimageable dielectric. A substrate 10 is coated with a photoimageable dielectric 35 by process 30. The photoimageable dielectric 35 is exposed in process 32 by irradiating with light of suitable frequency 31 through a mask 36. The photoimageable dielectric 35 is developed in process 34 to give patterned dielectric 39.
As can be seen, photoimageable dielectrics greatly simplify the process of fabricating circuits in multichip module, integrated circuit, liquid crystal display, and other microelectronic applications. Accordingly, it would be desirable to provide a photoimageable polyquinoline containing dielectric composition which could be photopatterned without the extra steps required by non-photoimageable dielectrics.